We demonstrate the interaction between surface acoustic waves and Dirac electrons in monolayer graphene at low temperatures and high magnetic fields. A metallic interdigitated transducer (IDT) launches surface waves that propagate through a conventional piezoelectric GaAs substrate and couple to large-scale monolayer CVD graphene films resting on its surface. Based on the induced acousto-electric current, we characterize the frequency domains of the transducer from its first to the third harmonic. We find an oscillatory attenuation of the surface acoustic wave (SAW) velocity depending on the conductivity of the graphene layer. The acousto-electric current reveals an additional fine structure that is absent in pure magneto-transport. In addition, we find a shift between the acousto-electric longitudinal voltage and the velocity change of the SAW. We attribute this shift to the periodic strain field from the propagating SAW that slightly modifies the Dirac cone.
References
The intrinsic carrier type and concentration are subject to the details of the sample preparation and surface doping. For CVD graphene on SiO2, the intrinsic density is smaller by a factor of at least 2, while the carrier mobility is higher. The different surface chemistry of GaAs and the resulting different number/types of trapped adsorbents between graphene and GaAs surface determine carrier concentration and mobility.
A resonance of 571 MHz for a wavelength of 8 μm corresponds to a sound velocity of about 4500 m/s. This is about 1.5 times larger than reported for ideal SAWs in GaAs. Such an enhancement of the SAW velocity by 50% has been reported for acoustic waves which also have considerable bulk components, [see e.g., Ruppel and Fjeldly, Advances in Surface Acoustic Wave Technology, Systems and Applications (World Scientific, 2001); J. Appl. Phys. 58, R1 (1985); J. Appl. Phys. 79, 8936 (1996)].
